THE MAGIC HORN FIX
There
are some tricks that are essential for eliminating horn "honk." The first is
to use the cone driver placed just below the horn, all the way up to a
frequency where it begins to "beam" due to the relationship of sound
wavelength and cone diameter. At a frequency where the resulting Q-factor
(directivity) of the cone matches that of the horn, the transition from cone
to horn will be smooth, and not abrupt-as it can be in systems where the
cone is too large and the horn is too small. If this condition is met, and
the frequency response of the cone is good well beyond the frequency up to
which it is used (a well-behaved upper end rolloff), then the horn will
enjoy a seamless transition from the cone and will not honk, assuming its
frequency response is good and uniform over its output angle. This latter
condition is referred to as being "power-flat" and is very important to the
transparent operation of the speaker system in rooms, with their concomitant
acoustic implications. If the speaker system is power flat, the sound in the
room will be as good as that particular room will allow it to be.
THE MIDRANGE
The midrange driver must be a cone, unless you live in a theater and don't mind a 4-foot high horn (I crossed my mid at 300 Hz into
the woofers). As it turns out a mid cone supplying 300 Hz to 1200 Hz gives
the proper effortlessness with very little power, and thus has extremely
small cone excursions and low distortion. As I mentioned earlier, I had to
trim the 2123H mid cone back 10 dB on the amp's gain control to get the
response through the band flat. The power absorbed by the mid cone driver
amounts to milliwatts most of the time, which helps to hold harmonic
distortion to very low levels, typically well below 1% THD up to dangerously
loud volume.
I experimented with a dozen midrange drivers before I was
confident that the 2123H with its high efficiency and limited excursion
linearity would produce sufficiently low distortion. It is a wonderfully
transparent driver and a large part of the reason this speaker system sounds
like listening to live music rather than loudspeakers.
(Note: as of 1995, JBL is manufacturing a new even higher power 10"
driver called the 2012H. If you can obtain 2012H's, performance will
increase even further.)
The driver is mounted on the baffle as close to the horn
as I could get it with my inexpensive mid chamber geometry. You could do
better if you are willing to cut the shape of the mid driver's frame into
the lower lip of the horn and snug the mid frame up into the cutout and, of
course, figure out a mid chamber arrangement that would clear the horn and
driver behind the baffle, but this is not measurably better than just a
touching fit.
The enclosure for the mid cone consists of a 10-inch
diameter concrete casting tube made of plasticized paper. Such tubes are
made by Burke Tube and Sonotube and no doubt many other regional paper
products manufacturers. The tube is mounted to the baffle by gluing into a
counter-bored shoulder cut-out, routed in the back of the baffle around the
mounting hole. The tube is about 12 inches long (deep), it is filled
completely but loosely with a "jelly roll" of unbacked fiberglass house
insulation cut from a roll about 4 feet long. The back end of the tube is
sealed air-tight with a disc of 1-inch thick medium density fiber board-the
same material used to build the rest of the box.
Please, even if you hate handling fiberglass, don't
substitute other absorbing materials for it. Fiberglass is unique in its
physical properties and substitutes will not work as well. Just get some
long heavy rubber gloves to handle the stuff, and shower off with cool or
cold water when you're done.
One letter I received inquired about using the JBL
three-inch throat midrange compression driver and horn. The horn itself is
44" wide by 42" high and with the driver attached, is 42" deep and weighs 82
pounds. The letter also asked about horn-loading the two 2227 cone drivers
for greater efficiency. Let me explain why I chose the geometry I did, so
that those of you inclined to even higher efficiency can decide how to
proceed from an informed set of criteria: First, one of my design goals was
the use of the typically small space behind the perforated theater screens
in the new smaller multi-cinema complexes being built around the country and
in Disney attractions that have such screen spaces. Even at Disney
Imagineering, it would have been impossible to argue successfully for the
space behind the screens required for horn loaded systems, and in fact, this
column design (once tested and listened to) proved that horn-loaded systems
were not necessary to play even the loudest ear-splitting explosion effects
in theaters of 200 or 300 seats. Second is the issue of acoustic impedance.
Simplistically, acoustic impedance is the ratio of radiation resistance to
the acoustical load. Radiation resistance varies with the size of the
acoustical aperture (source size) and the acoustical load is the air in the
room in which the source is operating. The source drives the load, and so if
we wish to avoid transmission line conditions where we must match the source
and load to obtain proper power transfer and flat frequency response, we
must provide a source of low acoustic impedance (small source size) so that
the source output is sufficiently robust to essentially ignore the load
conditions. What we lose doing this is some efficiency and sound pressure
level capability; what we gain is flatter frequency response and freedom
from such effects as the deterioration of performance when we move furniture
around or close or open a door or window. The real bottom line, however, may
be that this little column is simply more practical perhaps, than some other
designs.
© 1997 Drew
Daniels